1. Field of the Invention
The present invention relates to an optical head, an optical information apparatus and an optical-information reproducing method which records or reproduces information in or from an information recording medium such as an optical disk and an optical card.
2. Description of the Related Art
Conventionally, an optical head is known in which two objective lenses are provided (e.g., refer to Japanese Unexamined Patent Publication No. 2001-101692 specification). One objective lens is disposed on a radius line of an optical disk, and the other objective lens is shifted from the radius line. FIG. 14 shows a conventional optical head which is described in this Japanese Unexamined Patent Publication No. 2001-101692 specification.
In FIG. 14, an optical head apparatus 101 includes a first objective lens 102 and a second objective lens 103. These objective lenses 102, 103 are each used to reproduce an optical disk 106 of a different type. The first objective lens 102 is transferred, by a transferring means, on a straight line 104 shown by a broken line. The second objective lens 103 is transferred on a straight line 105 shown by a broken line. The rotational center of a motor 107 which rotates the optical disk 106 is located on the extension line of the straight line 105. The second objective lens 103 is disposed on a radius line of the optical disk 106. On the other hand, the rotational center of the motor 107 is not on the extension line of the straight line 104, and the first objective lens 102 lies off the radius line. FIG. 15 shows a photo-detector 110 which receives a beam of light that is reflected and diffracted after it has been concentrated on the optical disk 106 by the second objective lens 103. A light-receiving surface 111 of the photo-detector 110 is divided into four by division lines 112, 113. They are disposed perpendicular or parallel to a track-tangent directions 114 of the optical disk 106.
In Japanese Unexamined Patent Publication No. 2001-101692 specification, the method of generating a tracking signal is not specifically described. No concrete description is given about how to stabilize tracking control. Besides, according to the configuration of the division lines 112, 113, a tracking signal can only be obtained by a push-pull method or a phase-difference method. This presents a disadvantage in that a tracking signal has an offset when a lens is shifted, so that stable tracking control cannot be actually obtained.